Patent Documents 1 to 4 disclose techniques for life prediction of an engine using the number of rotations, output torque, power consumption, etc. of the engine. For example, in Patent Document 1, a sensor for detecting the number of rotations of an engine and a sensor for detecting the rack position detect the number of rotations of the engine and the rack position (a substitutional value of engine torque), a weight is determined on the basis of the detected number of rotations and rack position, the operating time of the engine is weighted by using the weight, and the life of the engine is calculated.    Patent Document 1: Japanese Patent Application Laid-Open No. 11-211622 (1999)    Patent Document 2: Japanese Patent Application Laid-Open No. 06-010748 (1994)    Patent Document 3: Japanese Patent Application Laid-Open No. 2001-011900    Patent Document 4: Japanese Patent Application Laid-Open No. 2005-171940